A number of important areas of technology, such as the fabrication of integrated circuits, solar cells, flat panel displays, etc., involve the use of chemical vapor deposition (CVD) techniques to deposit approximately uniform films over one or more entire surfaces of a work piece, such as a semiconductor wafer, or flat panel display. CVD typically employs a susceptor on which a work piece is placed within a CVD process chamber and then heated to temperatures high enough to induce thermal decomposition of one or more deposition precursor gases. In CVD, the film is inherently formed over the entire surface of the work piece since deposition is induced by the high (and necessarily fairly uniform) temperature of the entire work piece. Generally, work piece temperatures must be uniform to avoid excessive thermally-induced stresses within the work piece during CVD, and to ensure uniform composition and thickness of the deposited material. During device fabrication, in addition to the CVD processes outlined above, plasma-enhanced etch processes are typically also employed, where one or more entire surfaces of a work piece are etched, usually through openings in a patterned resist layer.
Many fabrication applications, such as nanotechnology, however, require a method for selective, or patterned, deposition or etching, i.e., a process in which only certain regions of a work piece will be deposited on or etched, not the entire work piece surface as in CVD or plasma-enhanced etching. Such processes require some method of local decomposition of the deposition or etch precursor gas. Laser beams, electron beams, and ion beams have all been used for these processes. For example, charged particle beams, such as electron beams, ion beams, or cluster beams, are used to deposit a metal by decomposing a precursor gas, such as an organometallic compound. Charged particle beams are also used to etch materials, using a precursor such as iodine or xenon difluoride, that combines with the work piece material in the presence of the beam to produce volatile byproducts. Charged particle beams can be focused to sub-micron spots, and can therefore be used to create precise, arbitrary structures by deposition or etching, by scanning the beam in a desired pattern while the beam etches or induces deposition.
A major problem with materials fabricated by beam-induced deposition is their purity is typically very low, often less than 50%, compared to that of materials grown by CVD. Numerous methods have been attempted to improve the purity of materials grown by beam-induced deposition. For example, Folch et al., “Electron Beam Deposition of Gold Nanostructures in a Reactive Environment,” Applied Physics Letters 66(16) pp. 2080-82 (1995), discusses the use of an environmental scanning electron microscope system to deposit gold nanostructures in a “reactive” environment comprised of a reactive gas other than the growth precursor, using an electron beam-induced deposition (EBID) process. An organometallic compound [Au(CH3)2(hexafluoroacetylacetonate)] served as the deposition precursor, and the work piece for film growth was indium-tin-oxide at room temperature. The deposition is performed in an oxidative environment containing either H2O vapor or a mixture of 80% Ar and 20% O2 (Ar/O2) (in the case of the latter, the Ar is a noble species which plays no part in the reaction). Folch et al. found that either of H2O or Ar/O2 reduced the carbon content of the deposited gold film, while pure Ar did not. The authors believe that carbon from within the growing gold film is removed through the formation of CO and CO2 molecules, which are volatile at room temperature, simultaneously with Au film growth.
Molhave et al., “Solid Gold nanostructure Fabrication by Electron Beam Deposition,” Nano Letters., vol. 3, no. 11, pp. 1499-1503 (2003) also employs an environmental scanning electron microscope to deposit gold nanostructures using [Au(CH3)2(hexafluoroacetylacetonate)] precursor in a water vapor environment. Molhave et al. found that the H2O environment enabled growth of polycrystalline gold cores (gold pillars created by the electron beam in their experiments). Molhave et al. also attempted unsuccessfully to grow gold cores in a reductive environment of 60% He/40% H2, and an oxidative environment of 80% Ar/20% O2. These results illustrate that not all gases that are reductive or oxidative can improve substantially the quality of the deposited film. Applicants' interpretation of the results published by Molhave et al. is that O2 and H2 have surface residence times and cross-sections for electron beam-induced dissociation that are too low to effectively purify the deposited structure by volatilising carbon (through the formation of either CHx or COx species in the case of H2 and O2 respectively). Molhave et al. and Folch et al. describe oxidative environments for assisting deposition.
A problem with prior art material purification methods such as those described above is that they reduce the volumetric growth rates of beam-induced deposition processes. This is a major shortcoming because the growth rates of beam-induced deposition processes are very low compared to other growth methods such as CVD. The low growth rates limit the applicability and usefulness of beam-induced deposition.
Sun, et al., “Electron-induced Nitridation of GaAs (100) with Ammonia” J. Vac. Sci. Technol., B, 11(3), pp. 610-613 (1993) describes using an ammonia precursor to form a film of GaN on a GaAs substrate under electron beam irradiation. Hubner et al., “GaN Patterned Film Synthesis: Carbon Depletion by Hydrogen Atoms Produced from NH3 Activated by Electron Impact,” J. Vac. Sci. Technol., A 13(4), pp. 1831-1936 (1995) describes a process for using trimethylgallium (TMG) and NH3, activated by electron impact, as precursors for GaN formation. The Hubner et al. process is performed at a temperature of 108 K, which was required to adsorb (with essentially an infinite residence time) a monolayer of TMG prior to electron beam irradiation. This low temperature is a substantial deviation from normal EBID processes occurring near room temperature and would be a limitation for industrial-scale applications of this process.
While water vapor and oxygen containing gases have been used to purify metal films deposited from organometallic precursors, such gases can oxidize the deposited material or the substrate, and they typically reduce material growth rates. Noble metals such as gold and platinum would not be expected to show oxidation effects; however many other films of interest would be expected to form deleterious surface oxides during any beam-induced deposition process employing oxygen.